The present invention relates to a device for soldering a small electronic element such as an IC chip to a substrate, for preventing the electronic element from becoming defective.
Recently, soldering using a light beam is used for soldering an electronic element because the size of the element becomes small. Terminals of an electronic element such as IC chip is connected to terminals formed on a substrate by a solder with a light beam.
FIG. 1 shows a conventional soldering method. An electronic element 3 is mounted on a substrate sheet 1 so as to position terminals formed on the underside of the electronic element 3 to wiring patterns formed on the substrate sheet 1. A push rod 4 is lowered to be abutted on the surface of the electronic element 3, thereby ensuring the positioning of the element 3 to the substrate 1. Then, solder 2 is deposited on the periphery of the element 3 and a beam 5 is projected on the solder 2. The solder 2 is melt and charged into the joint between the element 3 and the substrate 1. Thus, the terminals and wiring patterns are electrically connected to each other.
In accordance with the soldering, it is possible to solder the element to the substrate at a small hot spot. However, since the hot spot is very small, the heating is locally concentrated to raise the temperature at the connecting portion, causing thermal stress in the connecting portion due to thermal expansion.
Accordingly, when the solder 2 is solidified, a deformation 6 is formed on the substrate 1 as shown in FIG. 2. In the deformation 6, tension stress is produced in the joint to break the electrical connection between the substrate 1 and the electronic element 3 or the solder 2 is cracked to increase the resistance, causing bad electrical connection.
Even if these defects are not produced, such a deformation 6 causes a trouble in an automatic assembling process there-after. It is necessary to correct the deformation 6 to be flat. If a force is applied to the deformation 6 to flatten the substrate 1, the substrate 1 and the solder 2 are subjected to the force, causing the electrical connection to break and cracking the substrate 1.
The other soldering methods using heating flux also have these problems.